This invention relates to the chemical etching or chemical milling of metal parts, and is particularly concerned with a chemical milling process employing a unique and improved means for scribing the maskant on the metal or metal alloy employed in the process.
In chemical etching or milling, material or metal is removed from the surface of the part by treatment thereof in an etching solution to obtain a part having a desired structural or ornamental configuration. It is known to etch acid soluble metals such as iron and zinc with an acid solution such as aqueous nitric acid. It is also known to etch alkali soluble metals such as aluminum and its alloys with a solution having a solvent action on the aluminum or alloy surface, such as a hot aqueous alkali solution, e.g. one containing sodium hydroxide.
In many instances, in order to produce a desired etch configuration on an article in a practical manner, it is necessary to mask certain portions of the surface of the article so as to prevent contact of such surface portions by the etching solution.
Thus, presently practiced selective chemical milling of metal parts, such as aerospace parts, involves coating the part with a uniform layer of maskant material, manually scribing a pattern of "cut-outs" (which are to be subsequently subjected to chemical milling) using an overlay template to control the scribing pattern and a sharp instrument, such as an x-acto knife, to scribe or penetrate, the maskant, peeling away the maskant from within the circumscribed boundaries to expose the underlying metal, immersing the part for a controlled period of time in an etching bath, either acid or basic, to achieve localized thinning of the exposed metal, and removing the remaining maskant from the part surface.
The manual scribing operation represents the largest cost element in this process, including the labor involved in tracing the pattern as well as the expense involved in the provision of templates. Further, the presently practiced scribing procedure also presents a problem in that occasionally the underlying metal will be scored by the knife and result in increased local chemical milling attack, producing a furrow along the score mark after chemical milling, which must be blended out. Another important criterion is that it is important to be able to peel off the maskant following scribing, without disturbing the adhesion of the surrounding remaining maskant on the metal, e.g. aluminum surface. If there is any lift-off or peel-back of such remaining maskant, it must be repaired, for example tacked back down by wetting with solvent, so that subsequent chemical milling attack will not extend into the lift-off area.
It is known to utilize a laser for the purpose of removing metal or plastic from stock material.
Thus, U.S. Pat. No. 4,411,730 discloses a process for machining nickel-base super alloys wherein a thermal-effect process, such as a laser, is first used to remove metal, leaving a recast layer, followed by chemical milling wherein the etchant attacks and removes only the recast layer.
U.S. Patent No. 4,368,080 discloses a method of removing rust from the surface of a metal object by focussing a laser beam upon the rust to heat the rust to evaporation temperature to thereby evaporate the rust.
U.S. Pat. No. 4,405,852 discloses a method of decorating spectacle frame parts by removing a jacket material, which can be a plastic material, from the core according to a decorative pattern, thus exposing a bright surface of the core. The jacket material is removed by a laser beam.
An object of the present invention is the provision of an improved means for scribing a pattern in chemical milling maskant.
Another object is to provide a non-contact means of scribing a pattern in a maskant applied in the chemical milling of metals, such as aluminum and titanium, which avoids the disadvantages of manual scribing, and which does not deleteriously affect the base metal underlying the maskant.